On October 14, Ethereum reached a milestone that alarms many who have pushed for blockchains as “censorship-proof” technology. More than 51% of blocks produced in the preceding 24 hours were processed by relays that filtered out transactions involving Tornado Cash, a crypto mixing service that was added to the U.S. sanctions list in August.This 51% threshold doesn’t pose an immediate threat to Tornado Cash users, because even validators that censor transactions will still attest to the validity of blocks created by non-censoring validators. However, if 51% or more of validators were to also stop attesting to non-censored blocks, they would no longer be able to be added to the chain.